rtw88: avoid unused function warnings

The rtw88 driver defines emtpy functions with multiple indirections
but gets one of these wrong:

drivers/net/wireless/realtek/rtw88/pci.c:1347:12: error: 'rtw_pci_resume' defined but not used [-Werror=unused-function]
 1347 | static int rtw_pci_resume(struct device *dev)
      |            ^~~~~~~~~~~~~~
drivers/net/wireless/realtek/rtw88/pci.c:1342:12: error: 'rtw_pci_suspend' defined but not used [-Werror=unused-function]
 1342 | static int rtw_pci_suspend(struct device *dev)

Better simplify it to rely on the conditional reference in
SIMPLE_DEV_PM_OPS(), and mark the functions as __maybe_unused to avoid
warning about it.

I'm not sure if these are needed at all given that the functions
don't do anything, but they were only recently added.

Fixes: 44bc17f7

 ("rtw88: support wowlan feature for 8822c")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20200408185413.218643-1-arnd@arndb.de